The present invention relates to a heavy duty pneumatic tire of improved deflected wear resisting properties achieved by improving tread patterns.
Tires that can be used for all seasons are being desirable for use as heavy duty pneumatic tires that are employed for heavy vehicles such as trucks and busses. Such all-season heavy duty pneumatic tires are required to maintain high wet grip performances and steering stability performances for the aim of safe running on road surfaces of various conditions. For this purpose, block patterns are mainly employed as tread patterns in many cases.
On the other hand, it is the case with tires with block patterns, especially heavy duty pneumatic tires that large shear force acts on a first contacting side and last contacting side in a rotating direction during running or braking and sliding friction is generated between the tire and the road surface. So-called heel and toe wear (referred to as H/T wear) is accordingly apt to occur in which the first contacting side and the last contacting side in the rotating direction of the blocks are locally worn.
Japanese Patent Published Application 2004-106747 and Japanese Patent Published Application 2004-2032687 suggest providing tie bars between blocks as means for preventing such H/T wear. There has also been considered a means of prevention by setting a large land/sea ratio for increasing the rigidity of the tread surface. For instance, Japanese Patent Published Application 2000-177326 suggests a heavy duty pneumatic tire provided with shoulder blocks formed at shoulder portions being divided inside and outside in the tire axial direction and which is further provided with sipings including warped portions that warp in an outwardly convex manner.
With respect to tire performances, it is desired, in addition to the above performances, to restrict degradations accompanying progress of wear.
For instance, for restricting degradations in wet performances in a later term of wear, it is being suggested a) to form the tread rubber of a plurality of layers of different rubber compositions and b) to expose a rubber layer of high frictional force exhibiting wet skid performances on the tread surface in the later term of wear. However, problems are caused in such techniques that non-uniform exposure of the high frictional rubber layer caused through deflected wear. It will reversely harm the wet performances and causes a inter-layer peeling among the rubber layers.
In view of such circumstances, the inventors of the present invention have performed studies upon paying attention to compositional lengths of edges comprised by circumferential edges of the block portions (forming land portions) of tread patterns. It has consequently been found that the following points are important: (1) Edge components in the tire circumferential direction are considered to be particularly important in view of turning performances that are of special importance as wet performances. For maintaining such wet performances as high as those of new products, it is necessary to rather increase the edge component length in the tire circumferential direction accompanying the progress of wear;
(2) As for traction performances that are considered to be particularly important in view of on-snow performances, a) edge components in the tire axial direction are important and that b) it is preferable to increase such components accompanying progress of wear. However, on-snow performances are ensured up to 50% wear in case of winter tires. Accordingly, on-snow performance of tires that have exceeded 50% wear are not required to be maintained as high as wet performances of a new tire. Therefore, it would be sufficient to ensure tire axial edge components of not less than 0.5 times the edge component length of a new product;
(3) A ratio between edge component lengths in the tire circumferential direction and those in the tire axial direction should be balanced at respective stages of wear.
It has been found out that degradations in on-snow performances and wet performances accompanying progress of wear can be restricted low in these manners.
In this respect, Japanese Patent Published Application H10-76812 discloses a technique of restricting degradations in hydroplaning-resisting performances (wet performances) by increasing groove areas accompanying progress of wear.